In the data communication field, data is commonly transmitted asynchronously wherein the beginning of a group of bits of digital data are indicated by a start bit and the end of the data is indicated by one or more stop bits. High speed digital data links frequently utilize synchronous data transmissions such as the digital loop transceiver system taught in U.S. Pat. No. 4,432,089. Therefore, an interface must be provided whenever a synchronous data system is used in an asynchronous data environment.
Others have interfaced voice and asynchronous data and synchronous data in a two or four wire communication system by an oversampling technique utilizing a synchronous channel data rate which is at least twice the actual data rate. In the oversampling technique, a data sample is treated like a signal on a D.C. line. As a practical matter however, the channel data rate must be four or five times the real data rate to minimize channel edge jitter. Others have interfaced asynchronous and synchronous data by utilizing high speed synchronous protocols such as synchronous data link control (SDLC) or full high level data link control (HDLC) having control fields which identify when breaks between asynchronous bursts of digital data occur. However, these synchronous protocols require external intelligence and generally a synchronous channel bandwidth greater than the asyhcnronous data transmission rate. For example, using conventional means such as modems used with asynchronous to synchronous converters it is generally impossible to transmit asynchronous data at 9600 bits per second (bps) in a conventional synchronous 8 K bits per second channel typically provided by recently developed digital telephone networks.